One element of spacecraft design is attitude stabilization. Any uncontrolled body in space will tumble about all axes in responses to natural forces. A manned spacecraft not only has the effects of the natural forces, but additionally has to deal with the disturbance added by the human occupants. Tumbling is not normally acceptable because solar panels, communication antennas, and other instruments need to have a stable platform. Additionally, humans in a vehicle have adverse affects to the motion of an un-stabilized vehicle.
There are several methods to provide spacecraft stabilization; however, long duration missions or missions requiring precision pointing typically use an attitude control system in addition to thrusters. Attitude control includes three tasks: measuring attitude, correction of attitude, and a control law, which is the mechanism that determines the magnitude and direction of correction (e.g., see U.S. Pat. No. 7,014,150; U.S. Pat. No. 8,209,070). A typical control system uses momentum control devices such as reaction wheels or Control Moment Gyroscopes (CMGs) as attitude control, and an attitude control computer to execute the control law. Large satellites and space stations have momentum demands that usually require a CMG as the momentum device.
A CMG is generally used in spacecraft attitude control systems for 3-axis vehicle stabilization. A CMG generally includes a spinning rotor and one or more motorized gimbals that tilt the rotor's angular momentum. As the rotor tilts, the changing angular momentum causes a gyroscopic torque that through the principle of conservation of momentum results in the rotation of the spacecraft. Additionally, the change in angular momentum can absorb disturbance motion, thus stabilizing the spacecraft.
Contemporary space stations, including the Spacelab, Salyut, and International Space Station, have used a CMG for vehicle stabilization. An issue with using a CMG for larger space stations is that the momentum size for the space station must be known to set the momentum size required to stabilize and control the space station.
During the assembly of the International Space Station (ISS), a full size CMG array was sent into space with the first module of the ISS. The CMG array had an initial configuration, which far underused the initial momentum, but was “right sized” for the final ISS momentum. For any large space station constructed beyond Low Earth Orbit (LEO), the physical size of the CMG array required is so large that it cannot be transported in the space vehicles now in use. As such, current sizing methods require a “final” design of the next space station and do not allow the flexibility that is required for future NASA missions.